The present invention relates to data storage subsystems and apparatus which are capable of storing digital data, as well as image data; the invention relates to improvements in the construction and methods of operation of such subsystems and for enabling more readily identifying stored data and recovering from errors, particularly when such information signal recording apparatus is a buffered apparatus.
Historically, tape recording apparatus have been used to store sequential data sets and log-type data sets for digital data processors. In many instances, such tape record storage apparatus was highly interactive with data processing operations. In high performance data processing sites, such tape recorders employed vacuum column buffering of the tape for enabling the tape recorder to rapidly access data recording areas of the tape. Such a taper recorder is shown in U.S. Pat. No. 3,057,568 by Weidenhammer. Such tape recorders typically employed reels of tape having 2,400 feet of tape consisting of a magnetic coating on a polyester substrate. As such, the tape media were error prone. Typically, the tape media contained no physical identification on the tape except for recorded signals. Generally, the recorder received the signals from a host computer with each block of signals being of indeterminate length; that is, the host determines the number of signals to be recorded in each block; the length of each block being independent of the data storage subsystem. Utilization of tape recorders was typically a tape drive pooling arrangement; that is, when a spool of tape was mounted for recording or readback on a given tape recorder, single host usually communicated with that recorder for recording and reading signals on and from the mounted magnetic tape. In some logging operations, a plurality of hosts recorded on a single tape spool, hereinafter referred to as a tape volume; even then a given host would record its log data and then turn the recorder over to another host for the other host to record its log data. As such, the tape recorder was still in a pooling arrangement as opposed to a time sharing arrangement, as is commonly found with direct access storage devices. In accordance with this practice, it was not uncommon to find that a complete reel of tape would be read into data processing apparatus and then after suitable data processing functions on the data, the entire tape would be recorded. Generally, each tape volume was dedicated for a single purpose or for a single user. The term "user" means a process or a group of related processes being executed on a host processor.
Because of the aforementioned error prone media, once the signals are recorded on a tape, there is a low probability that an entire block of data can be lost due to imperfections in the media that occur after the recording; therefore, on a single tape volume of 2400 feet of tape, a previously recorded block of data signals can be entirely lost. When the data blocks are of indeterminate lengths, the only safeguard is to provide a header and a trailer label on the tape. Both labels would contain the number of blocks of data recorded on that tape. Then as the blocks are read off the tape, the count is kept to determine whether or not all the blocks were successfully recovered. Other techniques included the host assigning serial numbers to each of the data blocks, i.e., each data block will have a header area in which certain identifying data is recorded. Then the host, upon reading the data from the tape, requires its own programmed security procedures for insuring that all of the records were successfully recovered. Nothing in tape subsystems that were particularly adapted for recording blocks of indeterminate lengths provided facilities for enabling the host to analyze error situations in a more careful manner.
Some recording subsystems employed fixed length record areas. Such fixed length record areas can be large enough to accommodate most data blocks. If the fixed length data areas were not large enough to accommodate the data blocks, the segmentation of the data occurred. When such fixed length record areas are employed, the media usually contains permanently recorded or embossed area indicators. See U.S. Pat. No. 3,192,509. This arrangement increases the cost of the tape media. When tape media are employed primarily for archival purposes, an increased cost of the tape media is unacceptable; therefore, other means must be provided to economically insure a greater integrity of storage of signals when errors have occurred. Such enhancements are of particular importance to those recorders receiving blocks of data of indeterminate lengths.
Many tape recorders employing long lengths of tape in each tape volume will be subjected to various tape wrapping conditions; that is depending on the temperature and humidity of the environment in which the recorders operate, the tightness of the tape wrap can vary. Accordingly, it is difficult to precisely and repeatedly access a small area of the tape file in a reliable manner. It is desired, in conjunction with other factors discussed herein, to facilitate rapidly and accurately accessing an addressed small area in a tape volume.
It is also desired to reduce the cost of storage subsystems, particularly when such subsystems are used for archival purposes. Since vacuum column buffered tape recorders are expensive because of the cost of the tape vacuum columns, it is desired to use so-called "reel-to-reel" tape recorders. It is well known that the reel-to-reel tape recorders have notoriously poor access times compared to vacuum column tape recorders. In order to reduce the cost of tape recorders and yet provide the performance of vacuum column tape recorders, a large signal buffer is attached to the tape recorder. By promoting data using judicious motion techniques, the effective performance of a reel-to-reel tape recorder can approximate or better the access performance of many vacuum column tape recorders. Unfortunately, most buffers are volatile; that is, if power supply is removed from the buffer, the data is erased. Accordingly, techniques for reliably using a buffered reel-to-reel tape drive which facilitates recovery from power failure and other error conditions while enhancing access time to that of vacuum column tape recorders is needed.